This invention relates to a molten salt electrolysis process for producing an alkali metal using a binary alkali metal halide and strontium halide molten salt electrolyte.
The alkali metals, sodium, potassium, lithium, rubidium, cesium and francium are not found in elemental form in nature because of their high reactivity which causes them to combine with other elements to form various compounds. Electrolytic reduction is necessary to produce an alkali metal in its elemental form. The currently used process, on a world-wide basis, is the so-called xe2x80x9cDOWNS PROCESSxe2x80x9d, which was introduced in the early part of the 20th century for the production of sodium and lithium from their chlorides (Marshall Sittig, Sodium, Its Manufacture, Properties and Uses, American Chemical Society Monograph Series, Reinhold Published Corp., New York; Chapman and Hall, Ltd., London (1956) and Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Wiley/Interscience, New York (1997), Vol. 22, p. 327 to 354). The Downs Process uses a molten salt electrolyte consisting of a mixture of NaCl, CaCl2, and BaCl2 in order to reduce the melting temperature of the electrolyte to less than 600xc2x0 C. This makes the process more practical compared to using pure NaCl which has a much higher melting point of about 800xc2x0 C. The most important of the alkali metals, for industrial uses, is sodium. Sodium metal produced by the conventional Downs Process contains small amounts of calcium metal, which is co-deposited with the sodium at the cathode during electrolysis because the decomposition potential of calcium is close to that of sodium. Fairly elaborate and costly cooling/precipitation/filtration procedures are necessary to remove the calcium from the sodium metal produced by the conventional Downs Process. These purification steps remove practically all of the barium and most of the calcium. However, a significant amount of calcium is retained in the commercially produced sodium even after the purification process. Typical commercial sodium metal contains about 200 to 400 parts per million by weight (ppm) of calcium. For some applications (particularly nuclear, but also some chemical and electronic uses), an essentially calcium-free sodium is required. There are chemical routes to further purify sodium metal but these routes are extremely expensive.
Electrolytes which eliminated calcium and, therefore, produced calcium-free sodium were developed some years ago. These electrolytes are based on ternary and quaternary compositions containing NaCl, SrCl2, BaCl2 (ternary) and/or NaCl, SrCl2, BaCl2, NaF (quaternary) in near-eutectic proportions having melting temperatures of about 550 to 560xc2x0 C. (U.S. Pat. No. 2,850,442). A binary NaCl/SrCl2, electrolyte was also developed for the elimination of calcium from the sodium metal product (U.S. Pat. No. 3,119,756).
These electrolytes can be used in the standard Downs-type electrolytic cells without significant cell modification and/or change in operating conditions (same electrolyte/cell operating temperature of about 600xc2x0 C. and same cell voltage of about 7.0 to 7.5 V) because the melting temperature of these compositions is about the same as that for the standard NaCl/CaCl2,BaCl2 electrolyte. Current efficiency of cells operated with the ternary strontium electrolyte xe2x80x9cbathxe2x80x9d are in the range of 85% to 89%, similar to the standard NaCl/CaCl2,/BaCl2 bath. The quaternary bath can give somewhat higher current efficiencies but has the disadvantage that it attacks the brick lining of the electrolytic cells. Sodium made with the strontium-containing electrolytes is near calcium-free, but contains about 1000 ppm strontium and barium, which can be removed to low levels by a simpler conventional filtration process.
In the above electrolyte systems, cell operating temperatures are typically about 600xc2x0 C. when operating at high current densities in the order of 5 to 6 kA/m2. Current efficiency is reduced at higher temperatures, for example when operating at current densities higher than 5 to 6 kA/m2, due to formation of xe2x80x9csodium fogxe2x80x9d (emulsification of sodium in the electrolyte). This causes sodium losses and, therefore, losses in current efficiency when operating at higher current densities and temperatures above about 600xc2x0 C. in a Downs Process with the standard NaCl/CaCl2,/BaCl2 electrolyte. When in the form of sodium fog, sodium can partially migrate to the anode where the sodium is re-oxidized or reacts with chlorine to form NaCl. Sodium fog can also migrate to the surface of the molten electrolyte, which is exposed to air, whereupon sodium can also become re-oxidized.
Therefore, it is highly desirable to develop a process that is capable of operating at high current densities while producing substantially calcium-free sodium without a large increase in operating costs. An advantage of the present invention is the substantial elimination of calcium in the alkali metal product and increased electrolysis cell output without the need for substantial change in the cell design.
The present invention provides a molten salt electrolysis process for producing an alkali metal comprising carrying out the process using a binary electrolyte comprising an alkali metal halide and strontium halide, wherein the process is carried out at a high current density.
The present invention provides a process for producing an alkali metal by electrolysis from a binary molten salt electrolyte comprising an alkali metal halide and a strontium halide. In particular, the process is carried out at a high current density. The term xe2x80x9chigh current densityxe2x80x9d refers to about 7 to about 10, preferably about 7 to about 8, kA/m2. Current density is defined as the current in Amperes per unit area of effective electrode surface area. Increasing the current density on a given cell design can be carried out by increasing the current through the cell. For a cell of given design the electrode area is constant and an increase in current density means an increase in total current and, therefore, a corresponding increase in sodium production per unit time (provided there is no change in current efficiency).
The process of the invention can be carried out at any suitable temperature that can achieve an increase in sodium production per unit time. Generally, the temperature can be in the range of from about 605 to about 625xc2x0 C.
Any alkali metal halides can be used in the invention. The term xe2x80x9calkali metalxe2x80x9d refers to lithium, sodium, potassium, rubidium, cesium, francium, or combinations of two or more thereof. The presently preferred alkali metal halide is sodium chloride for it is widely used in electrolysis and sodium produced therefrom is commercially important. Similarly, strontium chloride is the preferred strontium halide.
The weight ratio of alkali metal halide to strontium halide can be any ratio so long as the ratio can produce a high purity alkali metal having calcium levels of less than about 50 ppm, barium levels of less than about 30 ppm, and strontium levels of less than about 600 ppm, preferably calcium levels of less than about 20 ppm, barium levels of less than about 10 ppm, and strontium levels of less than about 300 ppm. Generally, the ratio can be in the range of from about 35 NaCl/65 SrCl2 to about 19 NaCl/81SrCl2. For example, a binary electrolyte composition can contain 27 weight % NaCl and about 73 weight % SrCl2. Impurities in the electrolyte are preferably kept to less than about 2% by weight.
The process of the present invention for molten salt electrolysis of alkali metals uses a Downs cell design to carry out the process, originally disclosed in U.S. Pat. No. 1,501,756. A detailed description of this cell is given in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A24, VCH Verlagsgesellschaft, Germany, pp. 284-288 (1993). The pertinent portions of these documents are hereby incorporated into this specification by this reference thereto.
The ensuing discussion provides an example of a Downs cell useful in the present invention. However, it should be recognized that other designs based on a Downs cell can also be useful. The Downs cell will typically have a cylindrical brick-lined, steel casing. Cylindrical graphite anodes project upward through the bottom of the steel casing. Cylindrical steel cathodes project upward surrounding the anodes. In operation, the alkali metal is deposited on the inside surface of the steel cathodes and halogen gas is liberated at the graphite anodes. Typically, in a cell with four electrodes, the halogen is collected from the anodes into a gas dome from which the halogen exits the cell. The alkali metal rises in the electrolyte from the cathodes and is collected in a single compartment xe2x80x9ccollector ringxe2x80x9d covering all four cathodes.
To prevent back-mixing and reaction of the alkali metal and halogen, a hydraulically permeable diaphragm, such as a steel mesh screen, is used to separate the cathode and anode compartments.